1. Field of the Invention
The present invention relates to protection in an integrated circuit against electrostatic discharge (ESD). In particular, the present invention relates to integrating conductive polymer material on-chip to provide ESD protection.
2. Discussion of the Related Art
Mixing conductive and semi-conductive particles in an insulating polymer matrix can create a conductive polymer. In one instance, the conductive particles are aluminum spheres of several hundredths of a micron in diameter and each coated in a ceramic film several angstroms thick. Using an appropriate concentration of these conductive particles in the insulating polymer matrix, the particles can be spaced sufficiently close to each other to allow Fowler-Nordheim tunneling to occur. An example of such a polymer material, called xe2x80x9cSurgxxe2x80x9d, can be obtained from Surgx Corporation, Fremont, Calif.
The conductive polymer described above is insulating below certain electric field intensity (e.g. 10 volts per micron), but becomes highly conductive when the electric field intensity threshold is reached. FIG. 1 shows an electrical characteristic of a conductive polymer film. As shown in FIG. 1, as voltage is increased from 0 volts to about 5 volts (segment 103) across the conductive polymer film, the conductive polymer film is substantially insulating until threshold or xe2x80x9ctriggerxe2x80x9d point 100 is reached At trigger point 100, the conductive polymer film exhibits a xe2x80x9csnap-backxe2x80x9d phenomenon, in which the conductivity of the polymer film increases rapidly. As shown in FIG. 1, during snap-back, the voltage across conductive polymer film drops rapidly along segment 102, and thereafter, the current through the conductive polymer film increases substantially along segment 101. If the electrical field across the conductive polymer film is subsequently returned to a voltage below the snap-back voltage, however, the conductive polymer film returns to the pre-trigger insulating condition.
The present invention provides a method for creating a conductive polymer material on a semiconductor wafer. The conductive polymer material can be used to provide electrostatic discharge (ESD) protection in an integrated circuit.
In one embodiment of the present invention, the conductive polymer can be formed by: (a) applying on the surface of a semiconductor wafer a liquid film including polyamic acid, an organic solvent, and conductive particles; and (b) curing the liquid film at a sufficiently high temperature to cause polymerization of the polyamic acid and to evaporate the organic solvent to form a conductive polymer layer. The conductive polymer layer can be patterned by first providing a patterned masking layer over the conductive polymer layer; and etching the conductive layer in the presence of the patterned masking layer.
In an ESD application, the surface of the semiconductor wafer is first provided a patterned metallization layer prior to applying the liquid film. To provide ESD protection in a multilevel metallization system, a second layer of patterned metallization can be provided over the conductive polymer layer. In one embodiment including multilevel metallization, a dielectric layer is deposited over a first patterned metallization layer; and then openings are provided in the dielectric layer to form vias connecting metal lines in the first metallization layer to a subsequently formed second metallization layer. In that configuration, the conductive polymer material fills the via openings in the dielectric layer.
The concentration of the conductive particle in the conductive polymer material can be empirically determined. A sufficient concentration of the conductive particles is provided so that the conductive polymer becomes conducting when an electric field having an intensity above a predetermined threshold is applied across the polymer material.
In one embodiment, a passivation layer is provided above the conductive polymer layer. In one embodiment, the curing step is carried out by: (a) baking the liquid film at a first temperature over a predetermined time interval to create a polymerized layer without substantial cross-linking; and (b) baking the polymerized layer at a second temperature lower than the first temperature over a second time interval sufficient to eliminate the organic solvent. In another embodiment, after the organic solvent is eliminated, a hard curing step is carried out by baking the polymerized layer at a third temperature above the second temperature. The third temperature is selected to be sufficiently high to allow a cross-linking reaction in the polymerized layer to complete. If the polymer film is to be patterned, the hard curing step can be carried out simultaneously as the hard baking step in conventional processing of a developed photoresist layer.
According to the present invention, an ESD structure includes: (a) a first metal line provided on a semiconductor substrate for carrying an electrical signal; (b) a second metal line provided on the semiconductor substrate for carrying a supply voltage or a ground reference; and (c) a conductive polymer material in contact with the first and second metal lines. When the conductive polymer material becomes conductive when a sufficiently high voltage appears across the first and the second metal lines, such as when an electrostatic charge appears on the first metal line, the electrostatic charge is shunted to the supply voltage node or the ground reference. In one configuration, the polymer material is patterned. In another configuration, the conductive polymer material is provided as a blanket layer. In a third configuration, the first and second metal lines being provided on two levels of metallization. In a fourth configuration, the conductive polymer material fills openings in an interconnect dielectric layer. In that configuration, the openings connect metal lines in different levels of a multilevel metallization system. An insulating passivation layer is typically provided over the entire structure for mechanical protection.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.